The present invention relates to a coordinate-measuring machine, comprising a base for supporting a workpiece or a mechanical assembly to be measured, and a bar which is adapted to support a feeler device arranged to be carried into contact with the surfaces of the workpiece and which is movable with respect to the base along the three directions of a set of three orthogonal axes.
To support this bar and to allow it to be displaced in the three directions defined above, supporting structures of substantially two different types are utilized.
In a first structure the bar is fixed to a first carriage axially movable in a first vertical direction on a second carriage which, in turn, is movable in a second direction orthogonal to the first on a beam which forms part of a portal with a substantially U-shape form supported by the base of the machine. This portal is movable in a third horizontal direction orthogonal to the others, on suitable guides of the base.
In the other structure the bar is fixed to a carriage movable in a first horizontal direction on an arm, which is also horizontal, and in turn movable in a second vertical direction orthogonal to the first on a column supported by the base. This latter is movable on guides of the base in a third horizontal direction orthogonal to the above two.
In machines of the type briefly described there are various disadvantages.
First of all, the measurement precision is not very high nor constant in all working configurations of the machine; this disadvantage is due to the static deformations which are generated in some parts of the structure by the effect of the rather high loads which act on them and which are due to the weight of some of these movable parts, such as the carriages which support the bar to which the feeler device is fixed. The line of action of the resultant due to the weight of these movable parts is normally eccentric with respect to the mid-plane of the main support elements of the machine, such as the portal or the column, and therefore not only forces but also moments are applied to these: consequently some parts of the structure are stressed under flexion and torsion and experience deformations which considerably limit the precision of measurement obtainable with the machine.
Moreover, other measurement inaccuracies consequent on deformations of some parts of the structure are caused by the inertia forces which are generated during movements of the carriages or arm of the machine; such forces are particularly high because of the high mass of these members.
Thus, the mass of the movable parts of the machine being rather high, the accelerations of the parts themselves during their displacement must be maintained low in order to avoid transmitting high inertia forces to the supporting structure of the machine: consequently rather long displacement times are required to carry the feeler device from one working position to another.
Finally, the supporting structure of the machine described does not allow any variations in the configuration to adapt the machine to different working conditions: in fact, the feeler device can move only within a predetermined volume which is located above the support surface of the base of the machine, and which has rather small dimensions. Each machine, therefore, is only able to effect workshop measurements during the course of which the feeler device is carried solely in contact with the surface of a workpiece or mechanical assembly which has been positioned on the base of the machine. These machines cannot be adapted for use other than for this function, or for the testing of mechanical units on a production line, or mechanical units disposed on a support plane different from that of the base of the machine, or on mechanical units having very large dimensions.